1. Field of the Invention
The present invention relates to a dielectric filter for use in a high frequency band, a dielectric duplexer, and a communication apparatus using the dielectric filter or the dielectric duplexer.
2. Description of the Related Art
FIGS. 10A, 10B, and 10C show the configuration of a dielectric filter using a dielectric block, mainly for use in a microwave band. FIG. 10B is a front view showing the dielectric filter which is made to stand. FIG. 10A is a plan view thereof. FIG. 10C is a bottom view thereof. FIG. 10D is a left side view thereof. FIG. 10E is a right side view. In FIG. 10, a dielectric block 1 has inner conductor formed holes 2a, 2b, and 2c formed inside thereof, and inner conductors 5a, 5b, and 5c are formed on the inner walls of the holes 2a, 2b, and 2c. On the outer surface of the dielectric block 1, an outer conductor 3 is formed, and terminal electrodes 6 and 7 are provided in predetermined positions so as to be insulated from the outer conductor 3. The terminal electrode 6 is capacitively coupled to the inner conductors 5a, and the terminal electrode 7 to the inner conductor 5c. Like this, a dielectric filter composed of three-stage resonators and having a band-pass characteristic is formed.
However, in the dielectric filter as shown in FIGS. 10A to 10E, the terminal electrodes 6 and 7 input or output signals unbalanced with respect to the earth potential (the potential of the outer conductor) as a reference potential. Accordingly, in order to give signals to an amplifier circuit of a balanced input type, for example, it is needed that unbalanced signals are converted to balanced signals by means of a balun (unbalanced-to-balanced transformer). As a result, the area on a circuit substrate, occupied by a filter circuit part, is increased. This is one of the causes which make it difficult to downsize the dielectric filter.